(1) Field of the Invention
The present invention is generally related to a malfunction detection apparatus, and more particularly to an apparatus for detecting a malfunction in an evaporative fuel purge system which is provided in an internal combustion engine for temporarily adsorbing evaporative fuel, or fuel vapor, in an adsorbent in a canister and for purging the fuel vapor into an intake system of the internal combustion engine under given operating conditions, so that an air-fuel mixture is fed into a combustion chamber in the internal combustion engine.
(2) Description of the Related Art
Generally, the fuel vapor evaporated in the fuel tank is adsorbed by the adsorbent in the canister so as to prevent escaping of the fuel to the atmosphere. However, the amount of fuel adsorbed in the canister is limited because the capacity of the canister is limited. Therefore, there is a fuel vapor purge system that purges the fuel vapor adsorbed in the canister to an intake system of the engine in order to prevent overflow of fuel in the canister. The fuel vapor flows through a purge passage connecting the canister to the intake system of the engine and is purged to the inside of the intake system by a vacuum pressure generated by the engine operation. A purge control valve is usually provided to the purge passage to control the timing of the purging.
In this evaporative fuel purge system there is a possibility that the fuel in the canister overflows or that the fuel leaks to the atmosphere when a malfunction such as a fracture or a disconnection of the vapor line occurs. For this reason, an evaporative fuel purge system having a malfunction detection system is required.
In the Japanese Patent Application No. 3-138002, the applicant of the present invention suggested a malfunction detection apparatus for detecting a malfunction in an evaporative fuel purge system. In this apparatus, a negative pressure generated in an intake line of an internal combustion engine is introduced to a fuel tank and then the entire evaporative fuel purge system is put in a sealed condition. Existence/nonexistence of a malfunction is detected by monitoring a rate of change of the negative pressure inside the evaporative fuel purge system for a predetermined period of time. In the Japanese Patent Application No. 3-323364, the applicant of the present invention also suggested a malfunction detection apparatus in which existence/nonexistence of a malfunction is determined by monitoring a negative pressure inside an evaporative fuel purge system. In this apparatus, a bypass passage is provided between a vapor introducing hole of a canister and a purge passage, and a pressure sensor is also provided to a passage between the vapor introducing hole and a fuel tank. Specifically, existence/nonexistence of a malfunction is detected by monitoring a negative pressure detected by means of the pressure sensor when a control valve, provided to the bypass passage, is opened in order to introduce to the fuel tank a negative pressure generated inside an intake line of an internal combustion engine.
However, in the above mentioned malfunction detection apparatus suggested by the applicant, due to the introduction of a negative pressure generated inside an intake line, in addition to fuel vapor released from an adsorbent in the canister being purged into the intake line, fuel vapor from the fuel tank is also purged into the intake line via the canister.
Particularly in an internal combustion engine having an electronic fuel injection control system, a feedback control of an air-fuel ratio is performed so as to obtain the stoichiometric air-fuel ratio of the mixture to be suctioned into the engine. This feedback control is performed by correcting a basic fuel-injection time computed based on the rotation speed of the engine and the suction air amount (or a pressure inside the intake pipe) based on oxygen concentration in an exhaust gas as detected by an oxygen sensor provided in an exhaust pipe of the engine. However, despite the above mentioned air-fuel ratio feedback-control, the air-fuel ratio may temporarily be on the fuel-rich side of the stoichiometric ratio as a large amount of fuel vapor is suctioned into the intake line due to the introduction of the negative pressure.
Hence the above mentioned malfunction detection apparatuses suggested by the applicant cannot obtain an advantage of reduction in hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas performed by a catalytic converter because a large amount of fuel vapor is added to the basic fuel-injection amount due to the introduction of the negative pressure.